1. Priority Claim
This application claims the benefit of priority from European Patent Application No. 05001959.5, filed Jan. 31, 2005, which is incorporated by reference.
2. Technical Field
The present invention relates to a system and corresponding method for generating a wideband signal from a received narrowband signal, such as acoustic speech signals transmitted over a telephone system.
3. Related Art
The quality of transmitted audio signals often suffers from bandwidth limitations. Unlike face-to-face speech communication, that may take place over a frequency range from approximately 20 Hz to 18 kHz, communication by landline telephones and cellular phones is characterized by a substantially narrower bandwidth. For example, telephone audio signals, in particular, speech signals, are generally limited to a narrow bandwidth between 300 Hz-3.4 kHz. The audio components of speech signals that are lower and higher end frequency are simply not transmitted thereby resulting in a degradation in speech quality compared to face-to-face speech communications. This may cause problems in properly reproducing the speech at the receiving end and result in reduced intelligibility of the speech signal.
Several approaches have been taken to address such audio transmission problems. For example, several digital networks have been developed that have a higher speech transmission bandwidth than conventional telephone systems. Digital networks, such as the Integrated Service Digital Network (ISDN) and the Global System for Mobile Communication (GSM), have higher bandwidth speech transmission channels that allow for transmission of signal components with frequencies below and above the limited bandwidth of conventional systems. However, the higher bandwidth transmission channels result in a corresponding increase in network complexity and costs.
Other solutions have likewise been proposed to address the insufficiencies of narrowband speech transmissions. One proposed solution consists in combining two or more narrowband speech channels for the transmission of a single speech signal. However, this solution places significant demands on the telephone network and substantially reduces the amount of communications traffic that may be carried by existing equipment.
Another proposed solution consists in the utilization of speech codebooks at the receiver to construct wideband speech signals from received narrowband speech signals. In accordance with this approach, the receiver includes a narrowband codebook containing narrowband signal vector parameters and a corresponding wideband codebook containing wideband codebook signal vector parameters. The codebooks are generated to define the correspondence between narrowband and wideband spectral envelope representations of speech signals. In practice, an analysis of the received narrowband speech signal is used to select which of the narrowband signal vector parameters of the narrowband codebook provide the best correspondence with the received narrowband speech signals. The selected narrowband signal vector parameter is then used to select a corresponding wideband codebook signal vector parameter of the wideband codebook. In turn, the selected wideband codebook signal vector parameter is used to generate a wideband speech signal that corresponds to the received narrowband speech signal.
Even with the use of codebooks, the quality of the resulting wideband speech signals may be somewhat deficient. For example, abrupt changes from one entry of the narrowband member of the pair of codebooks to another may result in perceptible discontinuities and artifacts within the sequence of generated speech signals. Additionally, the number of wideband codebook entries may be limited and result in perceptible discontinuities in the generated wideband speech signal. Still further, the computing power required to execute such bandwidth extension methods is rather high, particularly when relatively large codebooks are employed. Thus, there is a need for improvements in systems that generate wideband acoustic signals from received narrowband acoustic signals.